A secondary battery, such as a nonaqueous electrolyte secondary battery, in which a lithium metal, a lithium alloy, a lithium compound, or a carbonaceous material is used as a negative electrode active material, is expected as a high energy density battery, and research and development is actively in progress. A lithium-ion secondary battery, which includes a positive electrode including LiCoO2 or LiMn2O4 as an active material and a negative electrode including a carbonaceous material allowing lithium ions to be inserted and extracted, has been widely put to practical use for a portable device.
On the other hand, when mounted on a vehicle such as an automobile or a train, materials having excellent chemical stability, strength, and corrosion resistance are required as constituent materials of the positive electrode and the negative electrode in terms of storage performance, cycle performance, long-term reliability of high output, and the like under a high temperature environment (60° C. or higher). Furthermore, when high performance is required in cold climates, high output performance and long life performance under a low temperature environment (−40° C.) are required. On the other hand, from the viewpoint of improving safety performance, a nonvolatile nonflammable nonaqueous electrolytic solution has been developed, but it has not yet been put to practical use because it involves a deterioration in output characteristics, low temperature performance, and long life performance.
As described above, when a lithium-ion secondary battery is mounted on a vehicle such as an automobile, high temperature durability and low temperature output performance become issues. For this reason, it is difficult to install and use a lithium-ion secondary battery in an engine room of an automobile as a substitute for a lead storage battery.
Since the electrolytic solution of the lithium-ion secondary battery is used at a high voltage of 2 V to 4.5 V, it is difficult to use an aqueous electrolytic solution. A nonaqueous electrolytic solution, in which a lithium salt is dissolved in an organic solvent, is used as an electrolytic solution for a lithium-ion secondary battery. It has been studied to improve high current performance and cycle life performance by improving the composition of the nonaqueous electrolyte solution. Since the nonaqueous electrolytic solution has lower ion conductivity than the aqueous electrolytic solution, it is difficult to reduce the resistance of the battery. In addition, the organic solvent, which is the solvent of the nonaqueous electrolytic solution, is easily decomposed at a high temperature and has poor thermal stability, causing a deterioration in the high temperature cycle life performance of the battery. From the above, although the use of a solid electrolyte for a nonaqueous electrolyte is studied, a battery having excellent high current performance cannot be obtained because the ion conductivity of the solid electrolyte is lower than that of the nonaqueous electrolytic solution.
On the other hand, a lithium-ion secondary battery including an aqueous solution for the electrolytic solution has low discharge capacity and low cycle life performance due to hydrogen generation from the negative electrode, thus making it difficult to put it into practical use.